It would be safer to do it in the reverse order, I think.
The fairness issue has different interpretations. If we are playing chess, then everyone starts out with exactly the same forces in exactly the same configuration (modulo left-right reversal), and that’s fair. But if we are playing bridge, then everyone gets a randomly chosen 13 cards, and before the cards are dealt, everyone’s chances of getting any one card are identical—and that’s fair too. It doesn’t change from fair to unfair just because you got a bad hand or a bad set of rolls.
On the other hand, my inclination is to give players a choice: either roll randomly and take what they get, or accept a fixed result that’s comparable to an average set of rolls and see what they can do with it. If they want to risk an inferior character for the sake of having a chance at a superior character, they have that option, but it’s not forced on them. My feeling is that there ought to be a little mean-variance trade-off in this: the fixed result ought to be just slightly less good than the average random result. This may be because I’m risk-averse and don’t understand the mindset of people who gamble for pleasure; I want an extra payoff before I’ll take the more random option.
The nonrandom approach is certainly better if you want to play a character who fits a concept. The random approach is more of a challenge: “Here’s a role, let’s see what you can make of it!” And you might not want that challenge.
It can certainly be really sucky if you create a random character and are faced with spending an entire campaign playing a character who has inferior traits, or just doesn’t fit the campaign theme. C has told me about playing in a Traveller campaign where the other characters were all merchants out to get rich, and she was playing an ex-soldier whose only skills were things like demolitions; her character spent entire sessions getting stoned in her cabin. The random rolls approach works better if you will make multiple random rolls and they have a chance to average out, which isn’t usually what we’re doing in an rpg, where you have some measure of long-term commitment to playing your character.
On the other hand, in terms of emulating the genre, superhero comics are famous for not having all characters be equal. The original Justice Society had Johnny Thunder, who was like Bertie Wooster but not as bright (his “superpower” was possession of a magical genie that he didn’t really know how to use); and Wildcat, a heavyweight boxer with no superpowers who was no mental giant; and the Atom, who started out at 5’1" and 98 pounds and then was physically trained to peak fitness, making him a match for much larger men. The Justice League was less frivolous, but still had both the nearly omnipotent Superman and the unpowered Batman, not to mention the classic Aquaman problem (“I’m more powerful than a locomotive!” “I can run faster than light!” “I can talk to fish!”), which has its own page in TV Tropes. So I don’t think getting a mediocre character falls short of emulating the genre as such. If it has problems, they’re more of the nature of “the conventions of the genre in the graphic novel medium aren’t satisfactory in the roleplaying game medium.”
I don’t have any a priori commitment here; I’m exploring conceptually what can be done to balance the objectives of “fair” and “fun to play” and “can emulate the genre.”